The present invention relates to semiconductor diode lasers. In particular, the present invention relates to high-power single-mode semiconductor diode lasers.
Many technologies employ single-mode high-power lasers. For example, erbium doped fiber amplifiers (EDFAs) use single-mode high-power pump lasers for optimal amplification of optical signals across optical fibers. Unfortunately, producing single-mode laser radiation is not always compatible with emitting laser radiation at high power. One of the reasons for this involves the width of the laser cavity. Single-mode radiation can be produced in a relatively narrow cavity, but the small size of the cavity limits the power output. If the cavity width is increased, thereby increasing the power output, multiple modes are produced.
FIG. 1 displays five known types of single-mode laser diodes, all of which exhibit varying degrees of strengths and weaknesses. For example, straight active region 101, which can be implemented in a ridge configuration or a buried configuration, is inexpensive and reliable. This configuration, however, incurs catastrophic optical damage when used to create 0.98 xcexcm radiation, and it leads to power saturation when used to create 1.48 xcexcm radiation. Thus, the configuration shown in 101 has inherent power limitations. Additionally, this configuration produces a highly divergent elliptical beam.
Master oscillator power amplifier (MOPA) 102 has a straight active region with a tapered output. This configuration can produce single-mode radiation at high power. It exhibits problems, however, in that it is complex to fabricate, has relatively low efficiency, and produces a highly-divergent beam with current-dependent astigmatism.
Unstable-cavity taper-emitting diode laser 103 exhibits a relatively high continuous-wave power. This configuration, however, has a high vertical and lateral divergence, and produces a highly divergent beam with current-dependent astigmatism. This configuration also exhibits a relatively low efficiency.
Diode laser with integrated beam expander 104 produces a low-divergence, non-astigmatic output beam. Unfortunately, the twin-waveguide structure exhibited in the figure is difficult to produce. In addition, this configuration exhibits additional losses and a decrease in output power.
Alpha-DFB laser 105 exhibits a very high single-mode power output. This configuration, however, is difficult to fabricate. Additionally, this configuration has a differential efficiency 25-30% less than for conventional laser diodes. Finally, this configuration emits beams with a relatively high beam divergence.
In general, those known apparatuses that include a narrow section and a tapered section (e.g., MOPA 102 and unstable cavity 103) do not allow for adiabatic coupling from the narrow part of the device to the tapered section. Without adiabatic coupling, additional modes are created. Furthermore, any time the coupling is nonadiabatic, the output beam""s lateral divergence depends only on the width of the narrowest part of the device; the lateral divergence can be as great as that for the straight active region devices, 105. Finally, these known devices exhibit current-dependent astigmatism.
Thus, a laser is needed that is relatively simple to fabricate, and that produces single-mode high-power radiation with a wide active region and a low beam divergence.
To alleviate the problems in the known art, an apparatus is provided that produces single-mode high-power laser light. In one embodiment of the present invention, a single-mode semiconductor diode laser comprises a single-mode section with a first width S0 that satisfies the equation S0 less than xcex(8 xcex94n n)xe2x88x921/2, a tapered mode-transformer section coupled to said single-mode section, and a power-supply section coupled to said tapered mode transformer section. The power-supply section in this embodiment has a second width larger than the first width. Additionally, the tapered mode-transformer section is characterized in that optical energy from the single-mode section couples adiabatically to the mode-transformer section and then to the power-supply section.